When running this code in Maple 13 and 16 this works just fine,
GenMs:= (k::posint)-> assign(m||(1..2*k) =~ seq([dx||i, dy||i], i= 1..k)):
GenMs(k), m || (1 .. 2*k);
gln := evalDG(l1 &s n1+`&s`(l2, n2)+sum('epsilon || i'*(cat(m, 2*i-1) &s cat(m, 2*i-1)+cat(m, 2*i) &s cat(m, 2*i)), i = 1 .. k));
But running the same code in Maple 2018 I get the following error:
Error, (in DifferentialGeometry:-Tensor:-SymmetrizeIndices) expected 1st argument to be a tensor. Received: `m2*i-1`^2
I don't know if it's related to Maple 2018 screwing with my tensor formatting:
For whatever reason I just noticed that my inline &t's have been turned into `&t` but that's a separate issue.